1. Field of the Invention
This invention relates to a method of forming a polymeric matrix containing inorganic filler material comprising bringing together filler material, polymerisable material and a catalyst to effect polymerisation of the polymerisable material and forming said polymeric matrix. The invention includes a filled polymeric matrix formed by the method of the invention, and it extends to vitreous filler material for incorporation into a polymeric matrix.
2. Description of the Related Art
Many polymerisable materials are known, and their use in more and more varied fields is widespread. One considerable advantage of such materials is that they can be used in the fluid or even viscoelastic state so that they may be shaped by moulding extrusion, injection or otherwise at a controlled temperature. Many polymerisable materials which are heat-formable at moderate temperatures or formable at ambient temperatures by such techniques require the presence of a polymerisation catalyst to initiate the chain reaction which gives a useful hardened formed article. In order that the polymerisation reaction can proceed properly to yield a homogeneous polymeric mass, it is of course necessary that the catalyst should be well distributed in the polymerisable material.
It is also well known to incorporate filler material into a polymeric matrix. This may be done in order to modify the mechanical, electrical or thermal properties of the polymer, or simply in order to reduce the cost of articles formed from the polymer. It is well known for example to incorporate glass fibres, whether individual glass fibres or glass fibre matting (which may be woven or unwoven), into a polymeric matrix. A filler material which is finding increasing use is vitreous beads. The expression `vitreous` is used herein to denote glass and vitrocrystalline material which latter is a material produced by heat-treating a glass to introduce a crystalline phase therein. The use of hollow glass beads as filler in particular allows the manufacture of articles of low densities.
Difficulties are encountered in achieving a good distribution of filler and catalyst in the polymerisable material for the formation of a high quality polymeric matrix, especially when the polymerisation reaction is one which proceeds rather rapidly. As an example of the difficulties may be cited the case of painted road markings which incorporate glass bead filler material to render the paint retro-reflective so that the marking may be seen more easily at night. One known technique is illustrated in U.S. Pat. No. 2,897,732 and consists in spreading a paint which is polymerisable to form a polyestervinylidene copolymer, spraying a powder polymerisation catalyst onto the surface of the paint marking and then sprinkling glass beads over the paint so that at least some of them can sink into the marking before polymerisation is completed. This technique suffers from a number of disadvantages. Firstly, it requires a rather complicated apparatus having three separate material discharge heads. Secondly, the catalyst, which is the most expensive ingredient, can easily be blown away during discharge and wasted. Thirdly, the catalyst is essentially deposited on the surface of the paint marking, and this gives rise to differential polymerisation of the paint leading to cracking of the surface and a lack of catalyst in the depth of the paint. Fourthly, although rapid polymerisation of the paint is obviously desirable, the more rapidly such polymerisation takes place, the less easy it is to achieve the desired distribution of filler beads through the depth of the paint marking.